1. Field
Embodiments described herein relate generally to an MRI (magnetic resonance imaging) apparatus and an MRI method.
Additionally, embodiments described herein relate to MRI technology to reduce “a value of a parameter having an upper limit” such as SAR (Specific Absorption Rate).
2. Description of the Related Art
MRI is an imaging method which magnetically excites nuclear spin of an object (a patient) set in a static magnetic field with an RF pulse having the Larmor frequency and reconstructs an image based on nuclear magnetic resonance signals (hereinafter referred to as MR signals) generated due to the excitation.
In MRI, an RF coil is used to transmit an RF pulse to cause nuclear magnetic resonance to a region to be imaged. The resonance frequency of the RF pulse is proportional to the intensity of the static magnetic field generated by the MRI apparatus. For example, in the case of a static magnetic field of 1.5 tesla, the resonance frequency is 63.8 MHz.
The RF pulse at this frequency causes an increase in temperature of the body of the object. Therefore, from the viewpoint of safety, the International Electrotechnical Commission (IEC) standard or other standards prescribe the upper limit of the energy of the RF pulse applied to the object.
Specifically, supposing that the energy of the RF pulse absorbed by 1 kg of living tissue is expressed in terms of SAR, it is prescribed that the SAR value for an arbitrarily set period of 10 seconds and the SAR value for an arbitrarily set period of 6 minutes do not exceed a first upper limit and a second upper limit, respectively. The upper limit of the SAR value varies depending on the region to be imaged, such as the whole body and the head.
In order to satisfy the safety standard concerning SAR, according to a conventional technique disclosed in U.S. Pat. No. 6,426,623, a sum of the energy values of the RF pulses transmitted to the object is calculated for each of the preceding 1 second, the preceding 5 seconds and the preceding 10 seconds.
And change of the pulse sequence is performed, if any of the following three cases occurs. A first case is that the sum for the preceding 1 second exceeds a first predetermined value. A second case is that the sum for the preceding 5 seconds exceeds a second predetermined value. A third case is that the sum for the preceding 10 seconds exceeds a third predetermined value. In the technique disclosed in U.S. Pat. No. 6,426,623, stopping of an RF pulse generator is given as an example of the aforementioned change of the pulse sequence. However, this may lead to cessation of imaging.
Thus, according to a conventional technique disclosed in Japanese Patent Laid-Open No. 2006-95278, the SAR value for the head or other regions to be imaged (referred to as a regional SAR hereinafter) is calculated before imaging. If the calculated regional SAR exceeds the upper limit, a warning message is displayed, and then, the pulse sequence is modified to prevent the regional SAR from exceeding the upper limit, and imaging is performed after it is verified that the dose to the object does not exceed the upper limit of the regional SAR.
The conventional technique disclosed in Japanese Patent Laid-Open No. 2006-95278 has preferred effects and advantages as described above. However, the modification of the pulse sequence that results in a reduction of the number of slices or the like is preferably as little as possible. This is because, for MRI diagnosis, imaging is preferably performed under optimal conditions that the number of slices is maximized as far as the SAR value does not exceed the upper limit, for example.
Therefore, there is a demand for a technique of reducing SAR so that imaging can be performed under optimal conditions.
In addition, there is a demand for a technique of reducing the value of a parameter other than SAR that has an upper limit.